Before Katrina, New Orleans had a murder rate 10 times worse than the U.S. average. The killers evacuated too. Tracing the criminal exodus.

On Aug. 29, 2005, New Orleans was on track to finish the year as the deadliest city in America, again. Crime had become atomized here--it was part of the culture, the air, the dark humor of the place. Under normal circumstances, criminologists believe, there are two ways to stop a cycle of gang violence: either dismantle the gangs or disrupt their business. In New Orleans, both happened overnight. Hurricane Katrina sundered what no man could, sending the criminals fleeing in all directions. So now there was a mystery: What would happen next? What would become of the criminal population when stripped of its neighborhood...